Unravelling the hidden benefit of Al incorporation on the stabilisation of sulphates during the catalytic fast pyrolysis of cellulose

Metal sulphate catalysts are widely used in biomass conversion to produce value-added chemicals, yet their stability in the reaction environment has not been studied. Most often, based on the inherent thermal stability of the pure species, sulphate catalysts are simply assumed stable when the reaction temperatures are low. However, in this paper, through studying the catalytic pyrolysis of cellulose ((C 6 H 10 O 5 ) n ) by experimental investigation, advanced characterisation and density functional theory (DFT) calculation, we prove that the ZnSO 4 - supported MCM-41 (with the inclusion of 1 wt% Pd in its matrix) is unstable at the pyrolysis temperatures of 400-450 °C, which is far below the thermal decomposition temperature of 646 °C for pure ZnSO 4 . This is due to the strong reaction between ZnSO 4 and formaldehyde (HCHO), an intermediate produced from the Grob fragmentation of the methyl group on glucose. It induces the loss of Brønsted acidity and reactivity of catalysts, along with the release of gaseous SO 2 that is environmentally concerning. Nevertheless, upon the inclusion of only 3.5 wt% Al into the MCM-41 matrix, ZnSO 4 was confirmed to remain stable during the cyclic and continuous tests, due to the formation of a strong covalent Zn-SO 4 -Al bond that enhances the dispersion of ZnSO 4 within the MCM mesoporous framework, and the energy demand for the desorption of SO 2 from the catalyst surface . Most significantly, such a hidden benefit of Al is complementary to its primary role in moderating the total acidity and Brønsted/Lewis acid ratio, leading to a record - high furfural (C 5 H 4 O 2 ) selectivity of 44-49.5% and a mass yield of 27.7-31.2 wt% from the pyrolysis of cellulose in batch-scale fixed-bed reactor. The hidden benefit of Al is also applicable to other sulphates, including CuSO 4 and Fe 2 (SO 4 ) 3 , although the extent for the improvement on their stability varies. These findings are expected to offer new insights for the design and use of sulphate-based acidic catalysts in practical applications. • ZnSO 4 @Pd-Al-MCM achieves furfural selectivity of ~49.5% and yield of ~31.2 wt%. • ZnSO 4 @Pd-MCM suffers from instability during cellulose pyrolysis (400-450°C). • HCHO strongly reacts with ZnSO 4 , driving SO 2 release and Brønsted acidity loss. • Al incorporation (3.5 wt%) forms Zn-SO 4 -Al bonds, enhancing sulphate stability. • Al-induced stabilisation is applicable to CuSO 4 and Fe 2 (SO 4 ) 3 sulphates.